Don't switch between doubles and ints in the caller, instead take doubles in
miPointerSetPosition and do the conversion there. For full feature we should
change everything down from here for doubles too.

Functional change: previously we'd restore the remainder regardless of
screen switching/confinement (despite what the comment said). Now,
screen changing or cursor constraints will cause the remainder be clipped
off. This should happen for cursor constraints but arguably not for screen
crossing.

This also corrects a currently wrong comment about miPointerSetPosition's
input coordinates.